1. Field of the Invention
The invention relates in general to a rotary torso, and more particularly to the improvement of a driving structure of a vertical rotary torso.
2. Description of the Related Art
There are various kinds of waist twisting instruments, and the swinging loop and the rotating plates are most common. In Taiwanese Patent No. 166930 (1991), a waist twisting and abdomen tightening body building instrument is disclosed. The driving structure of it consists of the follows. One end of it has a downward cam shaft which can rotate and scarf into the central bearing block of a base. The other end has a horizontal lever (rotating arm) in a bearing block of a upward cam shaft, and via a centrally downward cam shaft, a rotating plate can rotate freely and scarf into an upward bearing block at the other end of the lever. When users hold the handles installed at the protecting railing above the rim of the base, the feet stepping on the rotating plate and the waists twisting, the rotating plate rotates with the free end of the horizontal lever of the bearing block as the center. Meanwhile, the horizontal lever rotates according to body weights and the torque produced by the rotating force of the rotating plate. On the other hand, the rotating plate rotates eccentrically to the center of the base, and simultaneously rotates around the center of the base. This kind of theory of motion is very common in children's playing instruments in the scientific museum and the rotating cup in the fun fair.
However, to apply this structure in body building instruments for practical exercise, the rotating plate and the rotating arm are not related with each other, and rotate isotropically and irregularly with the axis of rotating plate and the rotating arm respectively. Therefore, the standing position of the users cannot be kept at the same location, and the body is and twisted. Especially, these two rotating parts trend to rotate faster and faster, thus cause the users hard to control and stand steadily, or even fall down. To prevent users to fall down, a three-way supporting abdomen tightening ring has to be installed to keep the body in balance. In addition, the above body building instrument cannot be folded, and this is inconvenient in storage and transportation.